Several kinds of networks are known in which plural nodes or stations gain access to a common transmission medium. Examples of such systems are single bus networks with collision detection, and token ring networks as well as token passing bus networks. Recently, networks providing another technique of multiple access to a common transmission medium have become of interest. These are systems comprising two parallel busses with counterflowing transmission of information of the two busses. Slots are released at regular intervals by headend stations, and these slots are used by the node stations for data transmission. Each node station has to request access to a slot by previously transmitting an access request in an Access Control Field of a passing slot. It keeps a count of access requests it has seen from other, stations (located upstream in request transmission direction) before it raises its own access request, and lets as many free slots pass by (for use by the other stations) as the count indicated, before it occupies the next free slot for transmission of its own data.
Such systems were described e.g. in a paper by R. M. Newman et al. entitled "The QPSX MAN", published in the IEEE Communications Magazine, Vol. 26 No. 4 (April 1988) pp.20-28; and in a Draft Proposed IEEE Standard 802.6 entitled "Distributed Queue Dual Bus (DQDB) Metropolitan Area Network (MAN)", Draft D6, Nov. 15, 1988.
Though these known distributed queueing systems are well suited for networks comprising a limited number of stations, they have some disadvantages which become unacceptable and may render the system inefficient when the number of stations is raised to several hundred, and when the length of the transmission busses is in the order of several kilometers.
These disadvantages are in particular: An "unfairness" for some stations with respect to others, due to the fact that each station has to await a free access request field before it can transmit a request, so that stations located upstream (in request transmission direction) are preferred; and an impossibility to guarantee the availability of a sequence of consecutive slots for one station desiring to transmit the portions of data packet without interruption.
In an article by S. B. Calo et al.: "Poll Actuated Multiple Access Technique for Broadgathering Systems", IBM Technical Disclosure Bulletin, Vol. 30, No. 1 (June 1987) pp. 72-75, a dual system is disclosed which has a plurality of stations connected between the busses. For controlling access to the busses, a pacer is provided at one end and a poller at the other end of the network. The pacing element partitions the time into contiguous slots which can be used by stations for data transmission. The polling element imposes a frame cycle structure by starting operation cycles. Each station can use only a limited number of slots in any cycle.
This method avoids the possibility that particular stations because of their topological situation can dominate the system (not leaving free slots for other stations). However, it has also disadvantages. As the allowed number of slots for each station is fixed or determined by central administration, situations may often occur that in a cycle a heavily active station cannot transmit as many slots as required, whereas momentarily silent stations will not use any slots of a cycle. This will result in a non-optimum utilization of the transmission network.